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“ However, if the Feruchemist were also an Allomancer, he might be able to burn his own metal storages, releasing the energy within them tenfold. Mistress Vin tried to burn some of my metals earlier, but couldn’t access the power. However, if you were able to make up the Feruchemical storages yourself, then burn them for the extra power…
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When a Metalborn has Allomantic and Feruchemical abilities in the same metal, they may store an attribute inside of a metalmind and then Allomantically burn the power. This gives a supercharged burst of the Feruchemical attribute, rather than the typical Allomantic ability.[2] For example, a Twinborn with both abilities in iron can store weight briefly in a metalmind and then burn the metalmind to draw out an order of magnitude more weight than was originally stored (instead of receiving the usual Pulling ability associated with the Allomantic burning of iron). Both Sazed[1] and Waxillium Ladrian[2] suggest that the Feruchemical power withdrawn is multiplied tenfold, but it is unclear how accurate this number is.

To understand the mechanics for this "hack", one must first understand the differences between Allomancy and Feruchemy. Allomancy is considered an end-positive system, meaning power is drawn from an outside source--in this case, from Preservation.[3] Feruchemy is an end-neutral system, with the power for the magic being drawn directly from the magic user--the amount of Investiture drawn out of a metalmind is equal to, and limited by, the amount of Investiture that was stored in the first place. In both cases, the type of metal acts as a focus, determining the form in which the magic manifests.[Citation needed] When Compounding is performed, the nature of the Investiture already stored in the metalmind determines the form taken by the magic--thus Feruchemical attributes are released.[Citation needed] However, because the Allomancy is fueled by Preservation, the amount of Investiture released is amplified. It is as if a previously hand-powered device were hooked up to an electrical cord to provide greater power output.[4]

The power of Compounding is even greater than it may appear at first glance because the Feruchemical attributes being released may also be stored, rather than used, and the process repeated with a greater reserve of Investiture at the Compounder's disposal. Repeated cycles allow for exponential (and effectively infinite) access to Feruchemical attributes.

While Compounding typically refers to Allomancy enhancing Feruchemy, there is most likely a way for Feruchemy to enhance Allomantic abilities as well.[5]

The Lord Ruler used this ability extensively, as he was both Mistborn and a full Feruchemist, so he could Compound in every metal. He would age briefly to store youth in atium and then burn it to effectively live forever.[1] He used gold Compounding to heal very rapidly, leading to rumors that he survived flaying and decapitation. Miles Dagouter also used gold Compounding to heal himself incredibly fast with an effectively limitless supply of health--hence his nickname, Miles Hundredlives.[2]Marsh is able to Compound atium due to abilities granted by his Hemalurgic spikes. Using the bag of atium taken from KanPaar, Marsh has remained alive by performing the same age trick as the Lord Ruler.[6]

During the Final Empire, Steel Inquisitors did not Compound, though it was sometimes possible for them to figure it out.[7][8] Compounding is one potential reason the Lord Ruler persecuted Feruchemists so extensively--he did not want Feruchemists to mix with Allomancers and create more Compounders.[1]

“ However, Feruchemy gives decreasing returns—it takes more than the proportionate amount of strength, for instance, to make yourself four times as strong as a regular man, as opposed to simply twice as strong. In the Lord Ruler’s case, this meant that he had to spend more and more youth to keep from aging.
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While the exponential use of Compounding is immensely powerful, it isn't without limitation. In some cases, the need to withdraw Feruchemical attributes may outpace even the ability to produce them using Compounding. For example, a gold Compounder with limited reserves may in some cases be unable to Compound health at the rate in which he needs to withdraw it. Likewise, the Lord Ruler could not have used Compounding to become immortal, because the rate at which one must withdraw youth increases with age. At some point his need to withdraw youth would outpace his ability to store and Compound it.

To perform Compounding, the metalminds themselves must be burned. Thus a person can only Compound so long as a supply of metal is accessible for the purpose of producing more metalminds. These metalminds also must be small enough to swallow or insert into the body, such that they can be Allomantically burned.

Compounding is also only possible with a metalmind that is not "locked" by Spiritual Identity--that is, the metalmind must either have been created by the user himself or it must be "unkeyed". Any Allomancer is technically capable of Compounding--even without Feruchemical abilities--if the metalmind has been "unkeyed" by means of Feruchemical Aluminum.[9]

According to rumor, Miles Dagouter would constantly tap his metalminds for healing. Waxillium took this to confirm his suspicions that Compounding could be dangerous to stop once started--presumably because the Compounder's body comes to rely on the stores provided. This suggests that Compounding often leads to savanthood.